1. Field of Invention
The present invention relates to an adaptive constant ON time adjustment circuit and a method for adaptively adjusting constant ON time, in particular to a circuit and a method for use in a DC-DC converter to improve transient ripple during mode transition.
2. Description of Related Art
Switching regulator can be classified into two types according to how its power switch is controlled: pulse-width modulation type and frequency modulation type. In a frequency modulation type switching regulator, a circuit controlling the power switch generates a constant ON time so that the power switch is turned ON for the same time period every cycle. The whole circuitry, by modulating the frequency, adjusts the level of an output voltage to regulate the voltage to a target value. Please refer to FIG. 1, wherein a buck switching regulator is taken as an example. In a prior art constant ON time switching regulator, an error amplified signal V1 is generated by an error amplifier EA which compares a feedback signal Vfb representing an output voltage Vout with a reference voltage Vref A comparator Com compares the error amplified signal V1 with a signal ViL representing an inductor current to determine a crossing point of the two signals, and generate a comparison signal Comp. Next, a one-shot pulse generator 15 generates a square wave signal PWM having the constant ON time. A driver circuit 13 controls the operation of a power stage 20 according to the square wave signal PWM to convert an input voltage Vin to the output voltage Vout, which is supplied to a load 30.
The prior art constant ON time switching regulator has a drawback that its response time is slow when the output voltage changes. Referring to FIG. 2, when the output voltage is shifted from high to low, the output voltage ripple generated in the prior art circuit can not converge in a tolerance band within a predefined settling time; instead, it is higher than the tolerance band and out of specification for a while after the predefined settling time. Similarly, referring to FIG. 3, when the output voltage is shifted from low to high, the output voltage ripple generated in the prior art circuit can not converge in the tolerance band within the predefined settling time, but is lower and out of specification for a while after the predefined settling time.
In view of the above, the present invention proposes an adaptive constant ON time adjustment circuit and a method for adaptively adjusting constant ON time for accelerating the response of the circuit during a transient period, to overcome the drawbacks in the prior art.